1. Field of the Invention
The present invention generally relates to a pressure pulse wave sensor and particularly to the art of reducing the size of the pulse wave sensor.
2. Related Art Statement
Pressure pulse wave, namely, oscillatory pressure wave produced from an arterial vessel of a subject reflects not only blood pressure of the subject but also physiological or pathological condition of the circulatory organ of the same. Hence, it is desired to non-invasively detect pressure pulse wave produced from an arterial vessel of a subject, for measuring blood pressure of the subject or making a diagnosis on the circulatory organ of the same.
In the above background, a pressure pulse wave detecting apparatus has been proposed which is of the type adapted to non-invasively detect pressure pulse wave produced from an artery underlying the skin of a subject, by pressing the artery via the skin. Such an apparatus is disclosed by U.S. Pat. No. 4,423,738 issued to Newgard. The disclosed apparatus includes a pressure pulse wave sensor adapted to be set on the skin of a subject. The pulse wave sensor includes a semiconductor substrate, a plurality of pressure sensing elements supported by the semiconductor substrate, and a pressing means for pressing the semiconductor substrate against an arterial vessel of the subject via the skin of the same. The semiconductor substrate includes a plurality of thin portions each obtained by locally forming a recess in one of opposite surfaces of the substrate which one surface contacts the skin of the subject when the pulse wave sensor is set on the skin. Each thin portion supports a pressure sensing element, namely, Wheatstone bridge including, for example, four piezoresistors. Each bridge detects strain produced in the corresponding thin portion as a result of transmission thereto of the pressure pulse wave from the artery, and converts the strain into electric signal (hereinafter, referred to as the pulse wave signal). The pulse wave signals generated by those pressure sensing elements are converted or processed by active elements provided on a wiring substrate to which the semiconductor substrate is secured. Such active elements include a multiplexer which multiplexes the pulse wave signals into multiplexed signal, a preamplifier which amplifies the multiplexed signal, and an analog to digital (A/D) converter which converts the amplified multiplexed signal into digital signal, which in turn is analyzed by a computer of a main device of the pulse wave detecting apparatus.
As is apparent from the foregoing, in the above described conventional pulse wave detecting apparatus, at least some active elements such as a multiplexer and a preamplifier are supported by the wiring substrate to which the semiconductor substrate is secured, in order to reduce the number of cables or lead wires used to connect between the pulse wave sensor and the main device. Consequently, the overall size of the pulse wave sensor which is set on the skin of a subject cannot be reduced to a satisfactory level. This leads to large possibility that the pulse wave detection is adversely influenced by contact of the sensor with an external object, or physical motion of the subject together with the sensor.